Close hashtree on application stop

Make sure to close the Yokozuna hashtrees properly during application
stop.

* Need to trap exits on `yz_index_hashtree` servers so that
  `terminate` callback will be invoked.

* Add `close_trees` call to `terminate` callback.

* Add `closed` flag to state to make sure trees are only closed if
  they are currenty open.
Merge pull request #118 from basho/rz-aae-close-tree

Close hashtree on application stop